1. Field of the Invention
The present invention relates to a method of determining characteristic correction data for correcting photoelectric signals detected from an original through an image sensor, so as to cancel influences of variations in photosensitivity and spectral characteristics between sensor cells, like CCD cells, and unevenness in illumination light amount on the original.
2. Background Arts
Recently, it has been known picking up image data from originals, such as picture frames on photographic film, through an image sensor, like a CCD image sensor, and using the picked up image data for making hard copies of the originals or for displaying still video images of the originals after processing and correcting the image data appropriately. In a linear CCD image sensor, the photographic film or other light-permeable original is moved at a constant speed in a sub scanning direction perpendicular to a line direction of the linear CCD image sensor, so the light passing through the original is detected by the image sensor, to pick up image data from the original. To pick up image data from reflective originals, like photographic prints or other printed matters, the linear CCD detects light reflected from these originals as the original is moved in the sub scan direction.
As well-known in the art, the CCD image sensor is constituted of a plurality of photoelectric conversion elements, called CCD cells. Since there are variations in photosensitivity between the CCD cells, as shown in FIG. 14, and it is hard to illuminate the original completely uniformly, as shown in FIG. 15, the signal levels from the CCD cells are not always equal even while the original is uniform in color and brightness, as shown in FIG. 16. Such unevenness in the output signals is called xe2x80x9cshadingxe2x80x9d. To prevent the shading from affecting the image data, the output signals from the image sensor should be corrected in accordance with the sensitivity variations between the CCD cells.
Specifically, regarding a film scanner for photographic film, a photo filmstrip having a base density in the whole are or a film having a reference light permeability is used as a reference original, and the shading is detected by scanning the reference original. On the basis of the CCD output signals obtained by scanning the reference original, that may be called reference data, shading correction data is determined for each CCD cell such that the reference data are made equal to each other when corrected with the shading correction data, and that the three primary colors are well-balanced in the image data of the reference original after the correction. Thus, by correcting CCD output signals detected from respective picture frames on photographic film with the shading correction data, the influence of shading is canceled. Regarding the image scanner for the reflective originals, a white sheet or the like is used as a reference original for detecting the shading and obtaining shading correction data.
According to a conventional method for producing such shading correction data, noise components are eliminated from the shading correction data by averaging reference data values of those pixels which are detected from the reference original by the same CCD cell of the linear CCD image sensor through 5 to 128 times of scanning of that reference original. In other words, the noises are eliminated by averaging data values of one color of those pixels which are located at the same position in the main scanning direction on the 5 to 128 main scanning lines.
However, the conventional method cannot sufficiently eliminate those noise components which result from stains, scratches and dusts on the reference original. Since these noise components are similar in size to the shading correction data obtained from the reference original, it has been difficult to distinguish the noise components from the valid data. Thus, the noises can appear as streaks in the image obtained through the linear CCD image sensor if the stain, scratch or dust on the reference original has a certain degree.
In view of the foregoing, an object of the present invention is to provide a method of determining characteristic correction data, whereby noise components resulting from stains, scratches and dusts on a reference original are well eliminated from output signals of the image sensor which are detected from the reference original. Thereby, the characteristic correction data may be determined based on the output signals detected from the reference original, without being affected by the noise components.
Another object of the present invention is to provide an apparatus for determining characteristic correction data by use of the method of the present invention.
According to an aspect of the present invention, in a method of determining characteristic correction data based on reference data values picked up through an image sensor by scanning at least a reference original a plurality of times per one photoelectric conversion element of the image sensor, wherein the reference original is uniform in density or in color and density, and the characteristic correction data is correcting characteristics of the respective photoelectric conversion elements, the method comprises steps of:
discriminating the reference data values between data of high spatial frequency components and data of low spatial frequency components; and
processing the high spatial frequency component data and the low spatial frequency component data differently from each other, to produce the characteristic correction data.
xe2x80x9cCorrecting characteristics of the respective photoelectric conversion elementsxe2x80x9d means correcting photoelectric signals detected through the respective photoelectric conversion elements of the image sensor, so as to cancel influences of variations in sensitivity between the photoelectric conversion elements, including variations in spectral characteristics between these elements, and unevenness in illumination light amount on the original.
Each of the photoelectric conversion elements picks up the reference data values from different points of the same reference original, or from a plurality of reference originals having the same properties.
According to a preferred embodiment, the discrimination step comprising steps of:
calculating a running mean value of the reference data values with respect to a respective one of the photoelectric conversion elements, to serve the running mean values as the low spatial frequency component data; and
subtracting the running mean values from the reference data values with respect to the respective one of the photoelectric conversion elements, to serve subsequent differences as the high spatial frequency component data.
The processing step preferably comprises steps of eliminating noises respectively from the high spatial frequency component data and the low spatial frequency component data; and thereafter adding the high spatial frequency component data and the low spatial frequency component data to each other in association with the respective one of the photoelectric conversion elements.
According to another aspect of the present invention, an apparatus for determining characteristic correction data comprises:
a reference data picking up device for picking up reference data values through an image sensor by scanning at least a reference original a plural number of times per one photoelectric conversion element of the image sensor, the reference original being uniform in density or in color and density;
a discrimination device for discriminating the reference data values between data of high spatial frequency components and data of low spatial frequency components;
a noise eliminating device for eliminating noise components from the high spatial frequency component data, the noise components resulting from noise factors existing on the reference original; and
a correction data calculating device that calculates a mean value of the plural number of data values of the high spatial frequency components after the noise components are eliminated, and a mean value of the plural number of data values of the low spatial frequency components with respect to the respective one of the photoelectric conversion elements, and then adds the mean values of the low and high spatial frequency component data with respect to the respective one of the photoelectric conversion elements, to produce the characteristic correction data for each photoelectric conversion element.
It is to be noted that xe2x80x9cfrequencyxe2x80x9d means spatial frequency in the following description.